To facilitate the connection of a conducting copper strip with wires of a series-parallel lighting string, a conventional connecting structure makes use of sharpened tips of a copper strip piercing the outer insulation of lead wires to reach internal metal conductors. For example, the structures disclosed in U.S. Pat. Nos. 5,051,877 and 4,777,573 have a construction in which semicircular grooves are provided on the connecting end of a husk of the lampholder and the corresponding cap to form wireways to secure the cord. However, to increase the variations in the twinkles of a lighting string, sometimes, it has to use two, three, or even four sets of lead wires; then, such a prior art structure must have a larger connecting end as well as a proportionally enlarged cap with three, four, or more semicircular grooves provided thereon. Consequently, a prior art connecting structure will lead to an oversize and overweight lighting string due to increasing functions of a lighting string and is not cost-effective.